Electroluminescent (EL) display is a self-emitting display mode which features excellent visibility (including high brightness, high contrast, very fast response speed and wide viewing angle), an extremely thin profile and very low power consumption. The EL display device itself emits light, as do cathode ray tubes (CRT), fluorescent and plasma displays. Unlike liquid crystal displays (LCDs), there is no need for backlighting. The response speed for EL can be as fast as 1000 times that for LCD, thus making this mode particularly well suited for use with moving images. EL displays may be used in a variety of applications, including aircraft and ship controls, automobile audio equipment, calculators, mobile telephones, portable computers, instrumentation, factory monitors and electronic medical equipment. Another major application for EL displays is as a light source, particularly as backlighting for small LCD panels in order to render them easier to read in low ambient light conditions.
EL displays work by sandwiching a thin film of a phosphorescent or other electroluminescent substance between two plates each of which comprises conductive elements in a predetermined pattern, i.e. electrodes, thereby forming addressable pixels on the display. The electrodes are formed as coatings either on the electroluminescent substance or on a separate support. Where the or each electrode is intended to transmit light, the electrodes are formed as translucent or transparent coatings, for instance using transparent conductive metal oxides. Equally, the or each support may be translucent or transparent, as required. Generally, at least the anode is transparent. The support generally functions both as a base for an electrode and as an insulating layer. The substrate also provides protection against chemical and physical damage in use, storage and transportation. Glass, as well as polymeric film, has been used as the insulating support.
EL display devices have utilised a variety of cathode materials. Early investigations employed alkali metals. Other cathode materials include combinations of metals, such as brass and conductive metal oxides (e.g., indium tin oxide). A variety of single metal cathodes, such as indium, silver, tin, lead, magnesium, manganese, and aluminum, have also been used.
Relatively recent discoveries in EL construction include devices wherein the organic luminescent medium consists of two very thin layers (<1.0 μm in combined thickness) separating the anode and cathode. Representative of OLED devices are those disclosed in, for instance U.S. Pat. No. 4,720,432.
When an electrical current is passed through the conductive elements, the electroluminescent material emits light. EL displays, being an emissive technology, rather than shuttering a light source as per LCD displays, are most useful in applications where high visibility in all light conditions is important.
The development of new, organic electroluminescent materials, which can produce the three primary colours with very high purity, has made possible full-colour displays with uniform levels of brightness and longevity. Polymers having such characteristics can be dissolved in solvents and processed from solution, enabling the printing of electronic devices. Conductive conjugated polymers are of particular interest. As used herein, the term “conjugated conductive polymer” refers to a polymer having pi-electron delocalisation along its backbone. Polymers of this type are reviewed by W. J. Feast in Polymer, Vol. 37 (22), 5017-5047, 1996. In a preferred embodiment, the conjugated conductive polymer is selected from:    (i) hydrocarbon conjugated polymers, such as polyacetylenes, polyphenylenes and poly(p-phenylene vinylenes);    (ii) conjugated heterocyclic polymers with heteroatoms in the main chain, such as polythiophenes, polypyrroles and polyanilines; and    (iii) conjugated oligomers, such as oligothiophenes, oligopyrroles, oligoanilines, oligophenylenes and oligo(phenylene vinylenes), containing at least two, preferably at least three, preferably at least four, preferably at least five, more preferably 6 or more repeating sub-units.
In addition to use in EL devices, such conjugated conductive polymers have been proposed for use in a variety of other electronic and opto-electronic devices, including photovoltaic cells and semiconductor devices (such as organic field effect transistors, thin film transistors and integrated circuits generally).
The present invention concerns the insulating and supporting substrate of an electronic or opto-electronic device comprising a conjugated conductive polymer, including an EL device (particularly an OLED), a photovoltaic cell and semiconductor devices (such as organic field effect transistors, thin film transistors and integrated circuits generally). The present invention is particularly concerned with the substrate of an optoelectronic device, particularly an EL device (particularly an OLED) or a photovoltaic device, and particularly an EL device (particularly an OLED).
The substrates can be transparent, translucent or opaque, but are typically transparent. The substrates are usually required to meet stringent specifications for optical clarity, flatness and minimal birefringence. Typically, a total light transmission (TLT) of 85% over 400-800 nm coupled with a haze of less than 0.7% is desirable for displays applications. Surface smoothness and flatness are necessary to ensure the integrity of subsequently applied coatings such as the electrode conductive coating. The substrates should also have good barrier properties, i.e. high resistance to gas and solvent permeation. A substrate for use in electronic display applications suitably exhibits water vapour transmission rates of less than 10−6 g/m2/day and oxygen transmission rates of less than 10−5/mL/m2/day. Mechanical properties such as flexibility, impact resistance, hardness and scratch resistance are also important considerations.
Optical quality glass or quartz has previously been used in electronic display applications as substrates. These materials are able to meet the optical and flatness requirements and have good thermal and chemical resistance and barrier properties. However, these materials do not have some of the desired mechanical properties, most notably low density, flexibility and impact resistance.
In order to improve the mechanical properties, plastics materials have been proposed as replacements for glass or quartz sheet. Plastic substrates have greater flexibility and improved impact resistance, and are of lighter weight than glass or quartz sheets of equal thickness. In addition, a flexible plastic substrate would allow the printing of electronic devices, for instance using the conjugated polymers referred to above, onto the substrate in a reel-to-reel process, which would reduce cost and allow the manufacture of curved-surface devices. However, the disadvantage of the use of polymeric materials is their lower chemical resistance and inferior barrier properties. Nevertheless, various barrier coatings have been developed to minimise this problem. These coatings are typically applied in a sputtering process at elevated temperatures. A barrier layer may be organic or inorganic, should exhibit good affinity for the layer deposited thereupon, and be capable of forming a smooth surface. Materials which are suitable for use to form a barrier layer are disclosed, for instance, in U.S. Pat. No. 6,198,217. In order to ensure the integrity of the barrier layer and to prevent “pin-pricks” therein, the surface of the polymeric substrate must exhibit good smoothness.
It is now possible to produce electronic display devices comprising barrier-coated polymeric materials which have greater flexibility and improved impact resistance, and are of lighter weight than glass or quartz sheets of equal thickness. However, some polymeric substrates undergo unacceptable dimensional distortion, such as curl, when subjected to the processing conditions, particularly elevated temperature, during the manufacture of display devices. It is desirable to provide polymeric substrates which exhibit good high-temperature dimensional stability during the high temperature techniques (such as sputtering) used to deposit the barrier layer. One such class of polymeric substrates is disclosed in the present Applicant's co-pending International Patent Application PCT/GB2002/04112.
In addition, the surface smoothness of a polymeric substrate is often inferior to conventional glass substrates. As noted above, surface smoothness is critical in order to ensure the integrity of the subsequently applied barrier and conductive coatings, and to avoid pin-pricks.
It is an object of this invention to provide a coated polymeric film substrate which overcomes at least one of the aforementioned problems. In particular, it is an object of this invention to provide a coated polymeric film substrate having improved surface smoothness, particularly wherein said substrate is suitable for use as a substrate, particularly a flexible substrate, in the manufacture of an electronic or opto-electronic device comprising a conjugated conductive polymer, including an EL device (particularly an OLED), a photovoltaic cell and semiconductor devices (such as organic field effect transistors, thin film transistors and integrated circuits generally). It is a further object to provide a polymeric film having improved surface smoothness, good high-temperature dimensional stability and high optical clarity.
As used herein, a device containing a conjugated conductive polymer preferably refers to an EL device (particularly an OLED), a photovoltaic cell and semiconductor devices (such as organic field effect transistors, thin film transistors and integrated circuits generally). As used herein, an opto-electronic device containing a conjugated conductive polymer preferably refers to an EL device (particularly an OLED) and a photovoltaic device, and particularly an EL device (particularly an OLED). As used herein, the term electronic device containing a conjugated conductive polymer excludes opto-electronic devices and preferably refers to semiconductor devices such as organic field effect transistors, thin film transistors and integrated circuits generally, and particularly organic field effect transistors.